Automotive vehicles presently use noble metal catalysts for the treatment of exhaust gases. Future vehicles may use such catalysts to process hydrocarbon fuels for fuel cell applications. But there remains a need for an improved dispersion of expensive noble metals on their carriers such as alumina carriers.
Vehicle exhaust systems use catalytic converters to treat unburned hydrocarbons (HC), carbon monoxide (CO) and various nitrogen oxides (NOx) produced from the combustion of hydrocarbon fuels in the engine. A typical catalyst comprises one or more noble metals dispersed on high surface area alumina carrier particles. Often the alumina particles are mixed with particles of another oxide, such as ceria or lanthana, for oxygen storage during exhaust treatment.
The catalytic converter for exhaust gas treatment then comprises a washcoat of such noble metal catalyst coated on the walls of an extruded ceramic body in the shape of an oval honeycomb, generally referred to as a monolith. The monolith comprises several hundred small longitudinal channels per square inch of its cross-section for passage of the engine exhaust gases in contact with the catalyst. The noble metal catalyst, typically contains platinum, palladium and rhodium and is called a three way catalyst because under suitable engine operation, it effectively reduces NOx and oxidizes HC and CO at the same time.
In order to have a more efficient and effective use of the expensive noble metal catalyst, the noble metal must be effectively and safely dispersed on a catalyst carrier particle such that noble metal particle surfaces are exposed to the exhaust gas. Activated alumina particles with a large surface area per volume are commonly used as the catalyst carrier material. To enhance its catalyst carrier properties, the alumina particles are often mixed with small amounts of other metal oxides, such as cerium oxide or lanthanum oxide. Since the dispersion of the noble metal is heavily dependant on interactions with these metal oxides as carrier particles, proper distribution of the metal oxides on the surface of the alumina is necessary in order to have a high surface area of the noble metal catalyst. Even though these catalyst systems are used in millions of vehicles, there is no indication that the noble metal is dispersed as effectively as it might be.
In a typical current practice, an aqueous slurry of mixed alumina particles and ceria particles, both greater than one micron in diameter, is prepared with sufficient fluidity to coat the many small cells of the cordierite monolith structure. The coating is dried and calcined on the walls of the monolith cells. The catalyst carrier particles are then impregnated with one or more solutions of noble metal salts. The noble metal solution impregnated carrier particles are dried and the monolith again calcined to decompose the noble metal salts and leave dispersed noble metal particles on the surfaces of the mixed oxides. While this practice is widely used, it has now been discovered that the noble metal may be more effectively dispersed on alumina/ceria particles by a new practice.
Thus, it is an object of the present invention to provide a method of forming a catalyst structure that will better disperse the noble metal on the surface of a catalyst carrier to improve catalyst performance while making efficient use of the expensive noble metal.